Hum reducing modulator



APril'29 l94T .1.` o. wELDoN 'A HUM REDUCING MODULATOR Filed NOV. 25,1942 y 1 high efficiency voperation signal frequencies.

Patented pr. 29, i947 HUM REDUCIN G MODULATOR James 0. Weldon, Del Rio,Tex., assgnor to Federal Telephone & Radio Corporation, New York, N. Y.,a corporation of Delaware Application November 25, 1942, Serial No.466.859

.'19 claims. (c1. 17e-171.5)

,This invention relates to Wave '.'transmission, and more particularlyto the reduction of disturbing interference, hum and distortion tendingto appear in thev output wave of an amplifying system. v

In a. carrier wave system utilizing a carrier Wave power amplifier atthe output of the system and having an audio frequency signal amplifierfor supplying amplified audio frequency modulating waves to the anode.circuit lof the power amplifier, the use of a lowfrequency alternatingcurrent for heating the cathode of theamplier, especially when theamplifier is of relatively high power, usually tends to cause modulationof the carrier wave at a plurality of hum frequencies corresponding tothe fundamental and several harmonics of thecathode heating current.These hum frequencies extend over an appreciable frequency range, suchas from 60 cycles to 300 cycles a second or higher, and thus may fallwithin the same frequency range as the desired signals, and

lead toserious complications and difficulties when l an attempt is madeto reducethem to a negligibly small amplitude.

If an attempt is made to eliminate these hum components by theV usualnegative feedback methods, many difculties arise which are notsatisfactorily overcome by ordinary eXpedients.

- One of the difficulties is to maintain stable operation of thev audiofrequency amplifying system without regeneration when .it is desired toprovide a wide band high quality signal transmitting system, as forexample in high fidelity'radio broadcasting. If the hum is opposed bythe usual negative feedback circuit, the signal suffers a correspondingloss'of amplitude within the hum frequency range, and at the same timeva further Jserious difficulty arises due to the tendency of systememploying amplification and subject tol interfering waves near theoutput of the system, and to reduce the amplitude of the undesired humor other interfering waves appearing in f the output, withoutsacrificing the quality of the wave transmission.

Another object is to counteract .the above difficulties and to provide awave transmitting system of relatively simple construction and of over aWide range of Another object is to reduce hum or other interferingcomponents produced in the output of a power amplifier by alternatingcurrent employed for heating the cathode of the amplifier.

Another object is to provide a substantially distortionless amplifiersystem having means for counteracting the effect of unwanted wavestending to be introduced near the output of the system.

Another object is to provide a negative feedback circuit for an audiofrequencyramplifier, for transmitting to the input of the amplifier oneor more vundesiredcomponents of hum frequency which have been introducedin the arnplifier, and to provide means for compensating for theresulting non-uniform frequency transmission of the amplier, so that theamplifier koperates substantially without distortion within the utilizedfrequency range including the hum frequencies, While effectivelycounteracting the near the output thereof, subject to interference,

is' provided with a first frequency selective negative feedback forsuppressing interfering components in the output of the system, a secondnegative feedback being coupled between the f input of the amplifier anda part of the system in advance of that-subject to interference, thesecond feedback having a frequency selective characteristic,- theeffectof which is complementary to that of the first feedback, so that thedistorting effect producedon the amplifier by the first feedback iscounteracted without destroying the interference reducing effect of thefirst feedback.

In another aspect, the invention provides an interference or humreducing negative-feedback circuit including a network having frequencyfamplitude and'frequency-phase characteristics adapted to improve thestability of operation of the amplier at vfrequencies outside theinterference frequency band, as compared with the operation of anamplifier employing prior forms fnegative feedback networks.

'.From another viewpoint, the invention providesv an improved form ofnetwork particularly adapted for use in a negative feedback for anamplierfor permitting of stable operation of the'amplifier whileproviding a frequency selective characteristic therefor.

From anotherviewpoint, the invention provides an amplifier system inwhich two negative 3 ing eects with respect to desired signal waves andhave non-complementary effects with respect to undesired interferingcomponents, while having similar frequency-phase characteristicsadapting the system for stable operation over a wide frequency range.

These and other objects and features of the invention will be understoodmore clearly in View of the following detailed description and claimsand the accompanying drawings in which:

Fig. 1 is a schematic diagram of the circuit of a carrier wavetransmitting system, such as a radio broadcast transmitter; and

Fig. 2 shows graphs of the transmission characteristics of networks Aand B of Fig. 1.

In Fig. 1 the audio frequency source I transmits signal throughtransformer 2, through the push-pull amplifier stage 3, 4, of theelectron tube type, through the additional amplifier stages 5,

through the output transformer 6 to the anode current supply conductor Iof the carrier wave power amplifier 8 of the electronic type withinwhich a carrier wave is modulated in well known manner in accordancewith the audio frequency signal wave transmitted to conductor -`I, there- .4

sulting modulated wave being transmitted to tuned circuit 9, coupled bythe line I4 to the antenna or other load I0. The carrier wave sourcesupplies waves to amplifier I2 which in turn excites the power amplifier8. The low frequency y auxiliary source of power I3 is connected in wellknown manner with the cathodes, not shown, in amplifiers 8 and I2, forsupplying heating current thereto.

The low frequency source I3 may be of the usual commercial frequency,such as for example as 60 cycles per second. As is well known, such alow frequency auxiliary current tends undesirably to modulate thecarrier wave and produce hum components in the envelope of the modulatedcarrier wave transmitted from power amplifier 8 to circuits 9, and I8.The hum components have frequencies corresponding respectively with thefundamental of the low frequency source I3 and harmonics thereof as highas the fifth harmonic or higher. Since these hum components represent aninterfering frequency within the same frequency range as that of thesignal waves from source I, the usual methods of avoiding interferenceare not adapted to eliminate the hum, because such methods ordinarilytend to discriminate also against the desired signal frequencies withinthe hum frequency range, thereby distorting the desired signal wave.

A portion of the carrier wave energy, having an envelope modulated inaccordance with the desired signal wave, is transmitted from the outputline I4 over the line I5 and ground to the primary of transformer I6,the secondary of which has its terminals connected respectively with theanodes I1 of the rectifier I8. The cathode I9 of rectier I8 is connectedthrough potentiometer resistances A2|) and 2| in series to a midtapconnection 22 on the secondary coil of transformer I6. Rectifier I8demodulates the carrier wave from the output of power amplifier 8, toreproduce the signal wave across the negative feedback line 23 whichconnects through'network B with the inputs of amplifier tubes 3 and 4,the network B being of such design as to cause the amplifier 3, 4, todiscriminate against `waves within the hum frequency range, thusproducing discrimination against signal wave components within the samehum frequency range and't'hereby distorting the signal'wave. In order tocounteract the distorting effect of network B on the operation of theamplifying system, a negative feedback circuit 24 is connected from theoutput of amplifier stages 5 with the input of amplifier tubes 3, 4, theinput of feedback 24 being connected to the system at a place in advanceof the coupling of stages 5 with the power amplifier 8 where the hum orother interfering waives are introduced. The frequency-amplitudecharacteristie of network A connected in feedback 24, is complementaryto that of network B and thus counteracts or compensates for thedistorting leffect of network B on the desired signal wave.

Since `the frequency-phase characteristic of the amplifier 3, 4, 5 withits interstage couplings and input and output connections, varies withthe frequency of the wave component being transmitted therethrough,difficulty arises in providing a feedback circuit 23 having such afrequencyphase characteristic as to maintain a proper value of negative.feedback to accomplish lthe desired results throughout the wide rangeof frequencies to be covered by the signal waves. Furthermore, any shiftin the phase angle of network B must be kept Within suchlimits that thefeedback circuit 23 can produce no appreciable positive feedback. Inorder to meet'these diculties, the frequency-phase characteristic ofnetworks A and B are made similar to each other within the range ofAfrequencies in which the amplitude of the feedback wave is sufficientto substantially affect the stabilityof the amplifier.

Further details of the circuit whereby the above results are obtainedwill now be described. An input circuitmay be traced from the controlgrid of tube 3 through the upper secondary coil of transformer 2,`through resistors 3U and 3| in series'to lthe cathode of the tube, thejunction between resistors .30 and 3| vbeing grounded at V32. Similarlyan input circuit may be traced from the control vgrid of tube 4 throughthe lower secondary coil of transformer 2 and resistors 33 and 34 inseries to the Acathode of the tube, the vjunction between resistors 33and 34 being also grounded at 32.

The output'of network B is connected with the ugrounded terminals of`resistors 3D and 33, so that the resistors 30 and 33 constitutecouplings between the feedback 23 and the input circuits of tubes 3 and4. The output terminals of network Aare .connected with the ungroundedterminals of lresistors 3| and 34, so that these resistorsconstitute'couplings between the feedback circuit 24 andthe inputcircuit of tubes 3 and 4. By providing separate coupling resistors forthe Anegative feedback circuits 23 and'24, respectively,

interaction between these networks is reduced to a minimumland'couplings 30 and 33 are relatively independent of couplings 3| and34.

Current is supplied to the anode of tube 3 through resistor 35 and tothe anode of tube 4 through'resistor 36. The amplified output wave fromtube 3 is transmitted from the anode thereof through the direct currentblocking capacitor 31., and-through resistor 38 andcapacitcr 39 inparallel, to the input Vof the amplifier stages 5. Similarly theampliiled'output wave from tube i4, is transmitted from 'the anode oftube 4 through-.the blocking capacitor 40, and through resistor 4I andcapacitor 42 in parallel, to the input of stages 5.

jnparallellwiththe primary of output transformer Bis connected a'circuit including the direct current blocking capacitor 43 thepotentiometer 44, potentiometer 45 and the blocking caneutralized at allfrequencies.

miams pacitor 4B all in series, the junction between potentiometers 44and 45 being grounded at 41 and the reduced voltage tap-off points 48and 49 be'- ing provided respectively on potentiometer 44 and 45, atwhich points are connected the conductors of the feedback .circuit 24.

Network A consists of a double, parallel T-network, that is,- oftwopairs of T-networks, each pair including two Ts in parallel, theupper pair consisting of the resistors 50, 5| in series with each otherin one line conductor of feedback circuit 24. The capacitor v52 beingconnected from ground to the junction of resistors 50, 5|. The other Tof the same pair includes the capacitors 53 and 54 in series in the lineconductor of the feed-back circuit; the resistor 55 being connectedin'rshunt from ground to the junction between the capacitors 53, 54. Inthe lower half of network A, a similar pair of Ts are provided, one Tincluding the series resistors 56, 51 and the shunt capacitor 58connected between ground and the junction of resistors 56, 51, the otherT including capacitors 25, 26 in series, the resistor 21 being connectedin shunt from ground to the junction between capacitors 25, 26. Thefrequency-amplitude characteristic of network A and the frequency-phasecharacteristic thereof, are shown in Fig. 2 which also shows thecomplementary frequency-amplitude characteristic of network B, thefrequency-phase characteristic thereof being represented by the samecurve as that for network A.

Network B includes a lattice network having four impedance arms 60, 6|;62, 63; 64, 65; and

66", 61; connected all in series with each other to form a closedcircuit. The -two non-adjacent junctions 68 and 69, provide inputterminals for tutes a Wheatstone bridge and the arms of the bridge orlattice are adjusted to balance according to well known practice inWheatstone bridges, so that when a voltage is applied across inputterminals 68, 69, no voltage appears across output terminals 10, 1I. Inother words, the coupling which normally exists between the input andoutput terminals by virtue of the arms connected therebetween, issubstantially completely Under these conditions, if the resistance ofarms 60, 6| be called R1; that of arms 62, 63, R2; that of arms 64, 65,

R3; and that of arms 66, 61, RA; the relation of the resistances of thearms when balanced may be represented as follows:

The feedback circuit 23 would therefore produce no feedback eect at anyfrequency, if the balanced elements R1, R2, R and R4 were the only`elements coupling terminals 68, 69 with ter--A minals 16, 1I.

In order to provide a frequency selective path through network B, afrequency differs from that of the rst connection. For this purposecapacitors 10' and 1I' are connected'in series with each other from thejunction of resistors 66, 61 in one arm to the junction of re-l sistors62, 63 in a non-adjacent arin' of tlie lattice. `Similarly a circuitincluding resistor 12, capacitor 13, capacitor 14 and resistor 15 inseries in the order named, is connected from the junction of resistors60 and 6| in one arm to the junction of resistors 64,v 65 vin anon-adjacent arm. The junction between capacitors 10 and 1| vand thejunction between capacitors 13 and 14 are grounded at 16'for the purposeof symmetry and balance of the network B with respect to ground,resistors 60, 61, 63 and 64 being preferably equal to each other, andresistors 6|,

66, 62 and 65 being also equal to each other, and

resistors 12 and 15 being equal to each other, capacitors 13 and 14being equal, and capacitors 10 and 1 being equal to each other. Thevalues of capacitors 10', 1I', 13 and 14 are so chosen with reference toresistors 12 and 15 as to provide paths in the lattice network havingthe frequency-amplitude and frequency-phase characteristic representedin Fig. 2. It will be seen that network B may be viewed in a slightlydifferent manner from a Wheatstone bridge, by considering the network tobe composed of twopairs of T-networks connecting the input to the outputrof network B. In one pair of T-networks, one T includes series lineresistors 60, 6| having at their junction a shunt connection to groundincluding resistor 12 in series with capacitor 13. The other T of thepair of T-networks, includes the resistor 61 in series with the resistor66, having at their junction the shunt connection to ground includingthe lcapacitor 10. The other pair,4 of T-networks includes one Tconsisting of resistors 64 and 65 in fseries, having the shuntconnection to ground from the junction of said resistors and includingresistor 15 and capacitor 14' in series, `the other T of the pair ofT-networks including resistors 63 and 62 in series and having a shuntconnection including capacitor 1| from there junction to ground.` Theresistors 12 and 15 serve to decrease the phase angle of the shunt path12, 13, 14, 15.

A high frequency inductive impedance `is provided in conductor I5 toreduce the amplitude of the carrier wave transmitted to transformer I6,the resistor 8| being connected from conductor I5 to ground. ConductorI5 is preferably arranged vas the inner conductor of the coaxial cable83, the outer cylindrical conductor of which is grounded and connectedwith one terminal of the primary coil of transformer I6, theotherterminal of the primary coil being connected with? conductor I5 throughthe D. C. blocking capacitor 84.

Sliding contacts 85, 86 are provided respectively on potentiometers 26,2| for adjusting the amount of voltage transmitted to the inputterminals 68, 69 of network B and for adjusting the balance of thefeedback circuit with respect to ground. Capacitors 81, 88 are providedin each side of the feedback circuit 23 for blocking the passage ofdirect current, resistors 89, 90 in series with the line conductors ofthe feedback circuit 23, serving to isolate or decouple network B fromthe circuit of rectifier I8. Equal small capacitors 9| and 92 in seriesand having their junction grounded, provide a path for the highfrequency carrier wave component of the wave being rectified in rectierI8, the grounded junction between the capacitors serving to balance theline conductors with respect to ground. Equalcapacitors 93 and 94 ofrelatively small values compared with capacitors 10', 1I",`13 and 14,provide a'shunt path for any high frequency carrier wave componentreaching network B from rectier'l'o, their reactances being so high asto have a negligible effect on the network. Capacitors 93 and 9dy are inseries in a circuit connected from the junction of resistors 00, l andthe junction of resistors 80, 65, the junction kof capacitors 93, 90being grounded.

While network B has been disclosed with a frequency selectiveimpedancepath 10', 1I'; and a further frequency selective impedance path l2, 13,'irland "l5, it will be understood that in some cases particularly wherea relatively small frequency range is covered by the hum frequencycomponents, only one frequency selective path, for example, 'i 'il' or12, 13, 14, 15, may sufce to `provide a frequency selective transmissioncharacteristic for the otherwise balanced, nontransmitting network B. Y

The anode current supply conductor 'l connects from the anode, notshown, of power amplifier 8, through the audio frequency choke coil 'l1and the space current supply source 'l5 to ground, the secondary coil oftransformiert being connected in shunt with coil 'li and having theblocking capacitor 19- in series therewith to prevent direct currentfrom passing through transformer 5.

A resistor 28 is connected between network A and the cathode of tubeit,Y and a resistor 20 is connected between network A and the cathode oftube 4 to isolate or Vdecouple the network from the other circuits ofthe tubes. A negative feedback circuitis connected from `the ungroundedterminal of resistor 30 through the capacitive reactancev95 in serieswith the resistor 96 to the anode or output circuit of tube 3, a similarnegative feedback circuit beingconnected from the ungrounded terminal ofresistor 33 through the capacitive reactance 9'! in seriesV with theresistor 98.130 the anode or output circuit of tube I4, for reducing thegain of theampliiier at the highest. signal frequencies and at`frequencies above the highest utilized signal frequencies to preventinstability and singing of the amplifier at very high frequencies. v

The parallel resistor 38 andcapacitor 39 and the parallel resistor M andcapacitor 42 serve to reduce the amplitudeof very low frequencies at thelower end of the signal frequency range to prevent singing at suchfrequencies in the form of oscillations sometimes referred to in the artas motor boating.

Resistors E0, 6l, 63, 50 serve to decouple the reactances of thefrequency selective paths J', ll; and 12, 73, id, 15; from the otherfeedback paths and circuits of the amplifier tubes 3, and 4.

vIt hasfbeen ,found that the above described circuits are especiallywell adapted to provide vstable operation of the amplifier whilesubstantially completely suppressing the-undesired components ofhumfrequency introduced` in theloutput ofthe power amplifier as a result ofthe cathode heating current acting magnetically orv otherwisey tomodulate the carrier wave current of the amplifier, at the same Ytimethelnetwork A serving to rrestore the characteristic of the amplifier to asubstantially fiat frequency amplitude characteristic throughout therange of the desired signal frequencies without destroying the humreducing effect of network B. For example, in a particular case where abroadcast transmitter was adapted to transmit modulating signal'frequency wavesv within arange extending fromapproximately 30 cyclesvper second-to 10,000

cycles per second, without objectionable departure from a flattransmission characteristic, interfering frequencies representing humcomponents extending from 60 cycles per second to as high as 720 cyclesper second at appreciable amplitudes were substantially completelysuppressed by the effect of network B on the amplifier, while network Asubstantially completely compensated for the distortion introduced intotheroperation of the amplifier by network B, without disturbing the humreducing effect of network B.

In the particular case just referred to, in network A, eachof theresistors 50, 5|, 50 and 51 had a resistance of 1,500 ohms; each of thecapacitors 53, 50,25 and 20 had a capacitance of 0.85 microfarad; andeach of the resistors 55 and 2i had a resistance of 600 ohms. In thesame case, in network'B, each of the resistors 00, 0l, 63, 60 had aresistance of 100,000 ohms; each of the resistors 5 l, 00, 02, 55 had aresistance of 50,000 ohms; each of the resistors 'l2 and 'l5 had aresistance of 2,000 ohms; each of the capacitors l0 'Il' has acapacitance of 0.05 microfarad; `and each of the capacitors 'i3 and 'Mhad a capacitance of 0.01 microfarad.v In this same case, each of theresistors 30 and 33 had a resistance of 25,000 ohms; each of theresistors 3l and 30 has a resistance of 2,000 ohms; each ofthe resistors28, 20 had a resistance of 2,000 ohms; each of the resistors and 90 hada resistance of 50,000 ohms; each of the potentiometers 20 and 2! had aresistance from ground to its respective slider 05 and 85,*ofapproximately 10,000 ohms; each of the capacitors 9|,

B is ymade-negligible, so that the desired negative feedback relationmaybe maintained over a wider frequency range of amplification than in thecase where an amplifier is provided with the usual form Vof frequencyselective circuit without any balanced Wheatstone bridge or latticenetwork that is balanced at all frequencies applied to the network. Thenetwork B is therefore particularly well adapted for use in negativefeedback circuits in general, apart from the specific combination withwhich it is used in the vabove described carrier Wave system.

Instead of regarding the network A as a corrective network to compensatefor the distortion produced in the amplifier 5, 4,5 as a result ofnetwork B, the network B may be regarded as a corrective element for thedistorting effect of netin the hum frequency range in amplifier 8 totheir normal amplitude relation to components outside the vhum rangewhile `preserving the improved signal-to-noise ratio.

Various modifications of the abover'described was - amplifier and saidpower amplifier for applying modulating voltages to said poweramplifier, a

rst negative feedback means coupling the output of said'power amplifierwith the input of said audio frequency amplifier, said first feedbackmeans including detecting means for producing an audio frequency currentcorresponding with the modulations of the high frequency carrierwave inthe output of said power amplifier, said first feedback means includinga network for selectively transmitting waves of said hum frequencies tothe input of said audio frequency amplifier for reducing the effectofsaid undesired hum modulation in said power amplifier, and secondfeedback means coupling the output of said audio frequency amplifierwith the input thereof and including means for counteracting thedistorting effect of said first feedback means on said audiofrequencyamplifier without destroying the hum reduction produced by said firstfeedback means.

2. In a wave transmitting system according to claim 1, the combinationin which the respective frequency-phase characteristics of said feedbackmeans are substantially equal to each other.

3. In a wave transmitting system according to claim 1, the combinationin which the respective frequency-attenuation characteristics of saidfeedback means are substantially complementary to each other.

4. In a wave transmitting system according to claim 1, the combinationin which the respective,-

frequency-attenuation characteristics of said feedback means aresubstantially complementary to each other and in which the respectivefrequency-phase characteristics of said feedback means are substantiallyequal to each other.

5. In a wave transmitting system according to claim 1,'the combinationin which an anode current supply conductor is provided for said poweramplifier and said coupling with the output of said audio frequencysignal amplifier iny cludes a coupling with said anode current supplyconductor.

6. In a wave transmitting system according to claim 1, the combinationin which said first feedback network includes a filter network of theWheatstone bridge type having four arms in balanced relation withrespect to the input and output terminals of the bridge at allfrequencies to -be applied to the bridge, and a frequency selecl tiveimpedance connected from an intermediate c point vin one of said arms toa point on another of said arms having a different potential from saidintermediate point in response to an alternating voltage impressed uponthe input terminalsA ofthe bridge.

7. In a wave transmitting system according to claim 1, the combinationin which said first feedf back network includes a filter having fourresistor arms joined in series with each other to forma closed circuit,two non-adjacent junctions 10 of said arms forming-input terminals forsaid filter, the othern two non-adjacent junctions vof said arms formingoutput terminals for said filter, eachof two of Asaid non-adjacent armsincluding two resistors joined in series and providing an intermediatetap at their junction, and

a frequency selective unit connectedbetween said two taps. y

8. In a wave transmitting system according to claim 1, the combinationin which said irstfeedback-networkincludes a filter having four resistorarms joined in "series with each other to form a closed circuit, twonon-adjacent junctions of said arms formingr input terminals vfor saidfilter, the other two non-adjacent junctions of said arms forming outputterminals for said filter, eachA said arm including two resistors joinedin seriesand providing an intermediate tap at their junction, afirst'unit having a predetermined frequency selective characteristicconnected between the taps of two of said non-adjacent arms, v

and a second unit having a different predetermined frequency yselectivecharacteristic from said 4first unit `connected between. the taps of theother two of said non-adjacent arms.

9.' In a wave transmitting system according to claim 1, the combinationin which said first feedback network includes a, filterA having fourresistor arms joined in series with each other to form a closed circuit,twonon-adjaoent junctions of said arms forming input terminals for saidfilter, the other two non-adjacent junctions of said arms forming outputterminals for said filter, each of two of said non-adjacent armsincluding two resistors joined in series., and providing an intermediatetap at their junction, and a frequency selective .unit includingv twocapacitors joined in series and having a ground connection at theirjunction, said unit being connected between said two taps.

, 10. In a wave transmitting system according to lclaim 1, .thecombinationinwhich said first feedback network includes a filter having'four resistor arms joined in series with each other to form a Closedcircuit, two nonadjacent junctions of said larms, forming'inputterminals forsaid filter, the other two non-adjacent junctions of Ysaidlarms forming output `termiiials for said filter, each said arm includingtwoV resistors joined in series and providing an intermediate tap attheir junction, a first unit'having a predetermined capacitanceconnected ibetween the taps of two of said non-adjacentarrns, and a,second unit including a predetermined resistance in series with acapacitance connected between thetaps of the othertwo of saidnon-adjacent arms.

11. In ay high frequency carrier wave transmitting system, acarrierwa've power amplifier of I the electron-discharge type having alow frequency auxiliary power supply circuit tending to produceundesired 'modulation Aof said carrier wave in said power amplifier athum frequencies,

- an audio frequency signal amplifier, a coupling between the output ofsaid audio frequency signal amplifier-and vsaidpower amplifier forapplying modulating voltages to said power amplifier, a first negativefeedback means coupling the output of said power amplifier with theinput of said audio frequency amplier, said first feedback meansincluding detecting means for producing an ,audio frequency currentcorresponding with the vmodulation of the high `frequency carrier wavein the outputof. said power amplifier,v said rst feedback .meansincluding a network for selectively transmitting waves of said hum fre-11 quencies to the input' of said audio' frequency amplifier forreducingthe effect offs-aid undesired hum modulation in saidA rpoweramplifier, and second feedback means coupling theY output of said audioVfrequencyv amplifier with the input thereof and including means forcounteracting the distorting effect of said first feedback means on saidaudio frequency amplifier without destroying the'v `hum reductionVproduced by said first feedbackjmeans, `and a'third negativev feedbackmeans coupling, the output of one'of the stages of the signal amplifierwith the input of one of the stages Afor reducing the gain of the higherfrequencies l2. In av high frequencyA carrier wave transmitting. system,a carrier wave power amplifier of the electron-discharge type having alow frequency auxiliary power supply circuit tending to produceundesired Vmodulation of said carrier wave in, said power amplifier athum frequencies, an audio frequency signall amplifier including at leasta` vfirst stage', a coupling between the output of said audio frequencysignal amplifier and said power amplifier for applying modulatingvoltages to said power amplifier, a first negative feedback f' meanscoupling the output' of said power amplifier with the input of said.audio frequency amplifier, said first Vfeedback means includingdetecting means .for producing' an audio frequency current correspondingwith the modulation ofthe high frequency carrier Wave in the output ofsaid power amplifier,- said first feedbackmeans including a network-forselectivelytransmitting waves of said hum frequencies to the input ofsaid audio frequency amplifier for reducing the effect of saidundesiredhum modulation in said power amplifier, and second feedback meanscoupling the output of said audio frequency amplifier with the inputthereof and including means for counteracting the distorting effect. ofsaid first' feedback means on said -audio frequency amplifier withoutdestroying thehumv reduction produced by said first feedback means, anda third negative feedback means coupling the output of the first stageof thesignal amplifier with the input.

13e. In aV` high frequency carrierk wave transmitting system, a carrierwave power amplifier of the electron-discharge type having a lowfrequency auxiliary power supply circuit tending to produce undesiredmodulation of said carrier wave in said power amplier at hum.frequencies, an audio frequency signal amplifier, a coupling between theoutput of said audio frequency signal amplifier and said power amplifierfor applying modulating voltages to said power amplifier, a

negative feedback means coupling the output of said power amplifier withthe input of said audio frequency amplifier, said feedback meansincluding detecting means for producingA an audio frequency currentcorresponding with the modulation of the high frequency carrier wave inthe output of said power amplifier, and said feedback means including afilter network of the Wheatstone bridge type for selectivelytransmitting waves of said hum frequencies to the input of said audiofrequency amplifier for reducing the effect of said undesired hummodulation in said power amplifier including four arms in balancedrelation with respect to the input and output terminals of the bridge atall frequencies to be applied to the bridge and a frequency selectiveimpedance connected from an intermediate point in oneof said arms to apoint on another of said arms having a different potential from saidintermediate point in response to an alternating voltage impressed uponthe input terminals of the bridge. f

11i-1in a high frequency carrier wave transmitting system, a carrierwaveV power amplifier of the electron-discharge type having Va lowfrequency auxiliary power supply circuit tending to produce undesiredmodulation of said carrier wave'in said power amplifier at humfrequencies, an audio frequency signa-l amplifier, a coupling betweenthe output of said audio frequency signal amplifier' and said poweramplifier for applying modulating voltages to said power amplifier, anegative feedback means cou-pling the output of said power amplierwiththe input of said audio' frequency amplifier, said feedback meansincluding detecting means forproducing an audio frequency currentcorresponding with the modulations of the high frequency carrier wave inthe output of said power amplifier, said yfeedback means including afilter network for selectively transmitting waves of said humfrequenciesto the input of said audio frequency amplifier for reducingthe effect of said undesired hum modulation in said power amplifierhaving four reistor arms joined in series with each other to form a'closed circuit,.two non-adjacent junctions oi" said arms forming inputterminals for said filter the cthertwo non-adjacent junctions of saidarms forming output terminals for 'said filter, each of two of saidnon-adjacent arms including two resistors joined in series Vandproviding an vintermediate tap at their junction, and a frequencyselective unit connected between said two taps.

l5. Ina high frequency carrier wave transmitting system, a carrier wavepower amplifier' of the' electron-discharge type having a low frequencyauxiliary power supply circuit tending to produce undesired modulationof saiolfcarrier wave in said power amplifier at' hum frequencies, anaudio frequency signal amplifier, a coupling between the output of saidaudio frequency signal amplifier and said power amplifier for applyingmodulating voltages to said power amplifier, a negative feedbackmeanscoupling the output of said power amplifier with the input of said audiofrequency amplifier, said feedback meansincluding detecting means forproducing 'an audio frequency current corresponding with the modulationsof the high frequency carrier wave in the output of saidpower amplifier,said feedback means including a filter network for selectivelytransmitting wavesof said hum frequencies to the input of said audiofrequency amplifier for reducing the effect of said undesired hummodulation in said power amplifier having four resistorarms joined inseries with each otheito form a ,closed circuitjtwo non-adjacentjunctions of said arms forming input terminals for said filter,l theother two non-adjacent junctions of said arms forming output terminalsfor said filter, each said arm including vtwo resistors joined in seriesand providing an yintermediate tap at their junction, a first unithaving a predetermined frequency selective characteristic connectedbetween the taps of two of said nonadjacent arms, anda second unithaving a different predetermined frequency selective characteristic fromsaid first unit connected between the taps of the other two of saidnon-adjacent arms.

16. In a high frequency carrier wave transmitting system, a carrier wavepower amplifier of the electron-discharge type having a low frequencyauxiliary power supply circuit tending 113 to produce undesiredmodulation of said carrier Wave in said power amplifier at humfrequencies, an audio frequency signal amplifier, a coupling between theoutput of said audio frequency signal amplifier and said power amplifierfor applying modulating voltages to said power amplifier, a negativefeedback means coupling the output of said power amplifier with theinput of said audio frequency amplifier, said feedback meansy includingdetecting means for producing an audio frequency current correspondingwith the modulations of the high frequency carrier wave in the output ofsaid power amplifier, said feedback means including a filter network forselectively transmitting waves of said hum frequencies to the input ofsaid audio frequency amplifier for reducing the effect of said undesiredhum modulation in said power amplifier having four resistor arms joinedin series with each other to form a closed circuit, two non-adjacentjunctions of said arms forming input terminals for said lter, the othertwo non-adjacent junctions of said arms forming output terminals forsaid filter, each of two of said non-adjacent arms including tworesistors joined in series and providing an intermediate tap at theirjunction, and a frequency selective unit including two capacitors joinedin series and having a ground connection at their junction, said unitbeing connected between said two taps.

17. In ya high frequency carrier wave transmitting system, a carrierwave power amplier of the electron-discharge type having a low frequencyauxiliary power supply circuit tending to produce undesired modulationof said carrier wave in said power amplifier at hum frequencies, anIaudio frequency signal amplifier, a coupling between the output of saidaudio frequency signal amplifier and said power amplifier for applyingmodulating voltages to said power amplifier, a negative feedback meanscoupling the output of said power amplifier with the input of said audiofrequency amplifier, said feedback means including detecting means forproducing an audio frequency current corresponding with the modulationsof the high frequency carrier wave in the output of said poweramplifier, said feedback means including a filter network forselectively transmitting waves of said hum frequencies to the input ofsaid audio frequency amplifier for reducing the effect of said undesiredhum modulation in said power amplifier having four resistor arms joinedin series with each other to form a closed circuit, two non-adjacentjunctions of said arms forming input terminals for said filter, theother two non-adjacent junctions of said arms forming output terminalsfor said filter, each said arm including two resistors joined in seriesand providing an intermediate tap at their junction, a first unit havinga predetermined capacitance connected between the taps of two of saidnon-adjacent arms, and a second unit including a predeterminedresistance in series with a capacitance connected between the taps ofthe other two of said non-adjacent arms.

18. In a wave transmitting system, an input circuit and an outputcircuit, wave amplifying means having means which introducesinterference in a limited band of low frequencies, a iirst negativefeedback means coupling said output circuit with said input circuitincluding a lter network for selectively reducing the gain of saidamplifying means in the limited band of low frequencies and therebyproducing distortion therein, and a second negative feedback meanscoupling said input circuit with a part of said amplifying system inadvance of said output circuit including a network filter for reducingthe effect of the distortion produced by said first feedback means inthe limited band of frequencies, and a third negative feedback meanscoupling said input circuit with a part of said amplifying system inadvance of said interference producing means for reducing the gain atthe high frequencies.

19. In a wave transmitting system for transmitting carrier frequenciesmodulated by frequencies within a certain total band, an input circuitand a carrier output circuit, wave amplifying means, a rst negativefeedback means selectively coupling said output circuit to said inputcircuit for reducing the gain of said system predominantly Within arestricted portion of said total band of frequencies and incidentallyintroducing both amplitude and phase shift distortion in said systemthroughout said total band of frequencies, and a second negativefeedback means coupling said input circuit with a part of saidtransmitting system in advance of said output circuit and havingattenuation and phase shift characteristics substantially completelycompensating for said distortion produced in said system by said firstnegative feedback means.

JAMES Oi. WELDON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,093,751 Witt Sept. 21, 19372,173,427 Scott Sept. 19, 1939 2,133,410 Wirkler Oct. 18, 1938 2,163,670Ditcham June 27, 1939 2,172,453 Rose Sept. 12, 1939 2,285,896 Brown June9, 1942 1,853,929 Rettenmeyer Apr. 12, 1932 2,058,210 Bode Oct. 20, 1936

